Messenger Conversation Feed

Hello,

Following an update or an unspecified manipulation on my part,
hundreds of Messenger conversations have disappeared from the chronological feed since Sunday or Monday.

By typing the names of my contacts, I can recover the dialogue, proving that they are not deleted but hidden. But I don't know where...

I have the dialogues from Friday to Sunday and those since the incident.
I hardly have any visible ones left (around 200 gone or more)!!

There remains visible a dialogue from:
Early August
A few from July
Then January 2018
and previous years
but mainly dialogues from inactive groups or profiles I haven't chatted with in years.
It's as if the Messenger conversations have been archived for all dialogues with contacts I regularly communicate with!!

I have many contacts in some of these dialogues only, without being able to remember the username, so it is important for me to find them.

I have tried everything:
FB settings
Messenger settings
Phone and app settings
I can't find anything.